The present invention relates to resistance welding electrodes and processes using welding electrodes, and more specifically to improved electrodes having selected shapes and welding current characteristics.
Resistance welding is a process used to permanently join metals such as steel sheet or other stock by typically pressing the stock together between a pair of electrodes, and then passing an electrical current from one electrode through the metal stock and into the other electrode. The electrical current involved is sufficiently high to cause sufficient heat due to electrical resistance to partially and momentarily melt the mating (or faying) surfaces of the metal stock at this point. In addition to this heat, sufficient pressure is applied to the stock by the welding tip faces of the electrodes to fuse the sheets of steel or other stock at this point into what is commonly called a weld nugget. Over time, this repeated heating and pressing operation causes breakdown, softening, mushrooming and other deformation of the electrodes. As this occurs, current requirements increase with the enlarged welding tip face contacting the stock until ultimately, replacement is required.
Furthermore, the steel being welded today is often galvanized, or coated with a zinc or other softer metal coating. This is especially true, for example, in the automotive industry. Under the heat and pressure of welding, this coating will tend to push aside and collect and also alloy with the electrodes which are typically a copper based metal. Both interfere with the welding process, but the alloying tends to compound the problems even more by further softening and increasing the electrical resistance at the surface thereby speeding up the breakdown and deformation of the electrodes. Due to the cost of replacing and/or refacing worn electrodes, it is the continuing desire of industry to increase the operational life of a resistance welding electrode at a reasonable cost. It is also the desire to minimize the electrical energy required to form a weld from both a cost and safety standpoint.
In a paper entitled NON-UNIFORM CURRENT DISTRIBUTION IN SPOT WELDING written by R. J. Bowers and T. W. Eagar of M.I.T., presented in Oct. of 1986 at the AWS's conference in Dearborn, Mich., the problem of electrode wear was addressed. Using finite element analysis, current density in different geometries of welding electrodes was computer modeled. The authors indicated that an optimal welding electrode geometry exists which balances two competing mechanisms of wear: uniform current distribution and mechanical/thermal stiffness. The suggested geometry, in FIG. 12 of the paper, is an electrode with a body portion, a nose portion having a welding tip face, and a concave, radiused taper from the welding tip face to the outer circumference of the electrode. The paper also concludes that electrode sheet interface angles approaching 90 degrees provide more uniform current distributions at the electrode face. However, the paper deals primarily with theory of geometric shapes, and focuses only on certain aspects of electrode design without addressing each of the many considerations in making a practical electrode, such as material selection.
As to material selection for such electrodes, this is addressed in U.S. Pat. No. 4,588,870 to Nadkarni et al., using a generally similar electrode shape to that disclosed in the M.I.T. paper above. In the Nadkarni '870 patent, it was demonstrated that conventional materials such as copper alloyed or otherwise combined with chromium, zirconium, cadmium, cobalt, nickel, beryllium, tungsten and/or molybdenum clearly cannot be used very well with the electrode shape disclosed in that patent. Such conventional copper alloys are reported to be severely softened by the high temperature, resulting in rapid mushrooming. In testing such copper alloys, Nadkarni '870 disclosed that on the 243rd weld both electrodes stuck badly to the galvanized steel and pulled off the adapters thereby evidencing failure. Instead, Nadkarni '870 indicates that success was obtained if the electrodes are formed of dispersion-strengthened copper rather than any of the conventional copper alloys.
However, dispersion-strengthened copper may require an extra fabricating step to cold form or upset the metal, and is quite expensive, typically several times the cost of conventional alloys. Accordingly, it is highly desirable to provide a welding electrode which takes advantage of the current density properties of electrodes as theoretically discussed in the M.I.T. paper while avoiding the difficulties of dispersion-strengthened copper caps and composite caps as required in the Nadkarni '870 patent. The present invention provides such a solution.
For example, dispersion-strengthened copper electrodes typically require significant conditioning early in the form of multiple runs before production welds can be made and before they attain any semblance of the desired generally-linear relationship between the number of welds and the weld current required. By reviewing the tables of the Nadkarni '870 patent, for early welds the rate of increase in weld current is fairly high for its electrodes up to about 500 welds. Only after this conditioning does the rate of increase in weld current tend to level out to a more linear function with a flatter slope.
It is a significant commercial desire to eliminate this need for preproduction conditioning of electrodes, and to have electrodes which exhibit a generally-linear weld current relationship throughout their life. This would be of significant value to industry, as welders could then make quality welds from the beginning without wasting production time conditioning each set of electrodes for several hundred welds. Also, welders are "stepped", or in other words, operated by increasing the electrical current amperage across the electrodes incrementally as the number of welds performed by the electrodes increases. During the life of the electrodes, they wear and may mushroom, resulting in an increasing surface area of the welding tip face as the number of total welds increases. Current stepping is done to maintain a generally constant current density at the welding tip face of the electrode by increasing the current to proportionally correspond to the increasing surface area of the welding tip face. Thus, stepping is used to maintain a generally constant current density. Unfortunately, with some prior electrodes, to maintain a generally constant current density suitable for welds, the rate of increasing current is not constant. However, the present invention provides a substantially-linear rate of increase of weld current, and thus it is easier for the operator to follow the optimal rate of current stepping thereby extending electrode life and reducing production costs.
The present invention provides these benefits. It reduces the amount of current required over more conventional electrodes for a given weld nugget, thus allowing energy savings and smaller welding equipment thereby reducing capital costs and space requirements which in turn may lead to more automation and reduced utility installation costs. Electrode life is extended as well, reducing replacement and/or refacing costs for electrodes. These advantages are provided while preferably avoiding the high cost of dispersion-strengthened copper. Also, the weld current characteristics of the invention are significantly improved, and provide for more uniform current stepping in welding processes to extend electrode life and improve operator efficiency.
These benefits are realized by a selected combination of electrode shapes, electrode metallurgy and electrical current stepping processes. In addition to the use of a concave profile taper such as that set forth in the M.I.T. paper discussed above, the present invention utilizes a specially-shaped welding tip face. More particularly, the welding tip face is fabricated within a selected range of convex curvatures which differ significantly in function and effectiveness from those disclosed in the Nadkarni '870 patent. Accordingly, contrary to the teachings of that patent, the present invention enables more conventional, less expensive copper alloys to be used successfully and greatly reduces or eliminates the need for electrode conditioning experienced with dispersion-strengthened copper. Also, the present invention provides a self-dressing function by mushrooming in a controlled manner. The present invention also provides a more linear weld current function which is continuously stepped up as welding progresses, thus providing a significant advance in the art.